The present invention relates to electrolytic capacitors containing means able to sorb the harmful substances created by such capacitors during their operation.
Known electrolytic capacitors, e.g. EDLC (Electrochemical Double Layer Capacitor) super-capacitors, are essentially comprised of an airtight casing, wherein electrodes typically formed of metal sheets are arranged, the electrodes being immersed in particular electrolytic solutions. The casing also contains elements having a gettering action for the sorption of harmful substances, and electrical contacts communicating the electrodes with the outside of the capacitor.
As to the electrolytic solutions, they are typically formed of a solvent and an ionic salt. In the case of EDLCs, for example, acetonitrile and propylene carbonate are often used as solvents, while tetraethyl-ammonium-tetrafluoroborate is often used as a salt.
During the operation of the capacitor, these solutions can create harmful substances, often in gaseous form, which can damage the capacitors even in an irreparable manner. Another possible source of harmful gases can be due to the desorption of some materials used inside the capacitor.
Carbon dioxide, carbon oxide and hydrogen are among the most harmful gaseous species; while water, which is another species being particularly harmful, is typically present in liquid form in the electrolytic solution.
The problem of sorption of harmful species inside the capacitors can be tackled by adding one or more sorbing elements mixed in the electrolytic solution, or by non-mixed sorbing systems. The use of materials with a gettering action being mixed in the electrolyte can be accomplished by liquid sorbers. Such type of solution is disclosed in International patent application PCT/IT2006/000349 of SAES Getters S.p.A. A second embodiment prescribes the use of solid sorbers being added to the electrolyte, as described in Japanese patent application publication JP 03-292712, wherein an additive including a particulate of platinum, palladium or alloys thereof is applied onto the sheets after these have been impregnated with the electrolytic solution. However, these sheets may have a very small thickness, in particular less than 10 micrometers (μm), whereby they may be damaged by the particles contained in such a particulate, due to their relatively large diameter, resulting in the risk of accidental short-circuits within the capacitor.
A system employing gas sorbers placed in delimited regions of the capacitor is described in Japanese patent application publication JP 2003-197487. In this case, the sorbing material is used in the form of sheets made of a polymeric material, such as polypropylene, supporting the sorbing material, such sheets being directly in contact with the electrolytic solution.
These types of solution are limited by the fact that the sorbing material, in addition to have the function of sorbing the harmful substances produced within the capacitor, must be compatible with the electrolyte, i.e., it must be completely inert with respect thereto, in order to prevent its sorbing properties from being jeopardized, or, even worse, that chemical species harmful for the correct operation of the capacitor are released as an effect of the reaction with the electrolyte. For instance, the possible decomposition of the gas sorber could vary the electric conductivity of the electrolyte. Such a chemical compatibility must be ensured by the sorber, even after the sorber has carried out its function by binding with the harmful species.